This invention relates to integrated optical components, and in particular to components for modulating a light signal with an electrical information signal for transmission over optical fiber facilities.
The guided-wave Mach-Zehnder interferometric modulator is a well-known optical device which has been extensively discussed in the literature in such articles as "Multigigahertz-Lumped-Element Electrooptic Modulator," by Richard A. Becker, IEEE Journal of Quantum Electronics, Vol. QE-21, No. 8, Aug. 1985, pp. 1144-1146 and "Guided-Wave Devices for Optical Communication," by Rod C. Alferness, IEEE Journal of Quantum Electronics, Vol. QE-17, No. 6, June 1981, pp. 946-959. The interferometric modulator consists of a single input waveguide, an input branching region for splitting the input light power into two substantially equal portions, two branch waveguides, an output branching region for recombining the propagating light power in the two branch waveguides, and an output waveguide. By effecting a phase shift in one branch waveguide relative to the other, the combined output light power is between zero and the input power level, depending upon the magnitude of the phase shift. Such phase shifts are effected by means of electrodes disposed on the substrate of the optical waveguide proximate to one or both of the branch waveguides. When a voltage is applied, the electrooptic effect changes the refractive index of the proximate branch waveguide changing the optical path length, thereby effecting a phase change in the branch. By keeping the branch waveguides sufficiently apart to prevent optical coupling between the branches which would degrade performance, voltage variations are linearly transformed into the phase changes and thus into amplitude variations in the light output power level. Accordingly, by modulating the electrode voltage with an analog or digital information signal, the output light power is similarly modulated and can be coupled onto a fiber waveguide for transmission.
In the prior art interferometric modulators, either a two or a three traveling wave electrode configuration is employed. In the two-electrode configuration, the electrodes are disposed along one branch waveguide length. Advantageously, the electrodes can be impedance matched to their driving circuits by selecting the electrode widths as a function of the gap between electrodes. By impedance matching the electrodes, no power is lost to reflections. Disadvantageously, however, the available power effects a phase shift in only the one associate branch waveguide thereby limiting the depth of achievable modulation for a given voltage.
In the three-electrode configuration one electrode is commonly disposed between the two branch waveguides and separate grounded electrodes are disposed along each branch waveguide. A voltage between the common electrode and each ground electrode effects an equal and opposite phase shift in each branch waveguide thereby achieving twice the net phase shift than the two-electrode configuration for the same voltage. This plus-minus phase shifting arrangement is known as push-pull and is advantageous for its efficient voltage utilization in that for a given voltage, twice the net phase shift is effected than in the aforenoted two-electrode configuration. Disadvantageously, however, because of the need to keep the branch waveguides far apart to prevent optical coupling, the three-electrode configuration cannot be impedance matched to the driving circuits, thereby resulting in microwave reflections and losses and thus not fully efficient use of the available power.
For high speed operation neither configuration is voltage efficient. Whereas push-pull operation is achievable in the three-electrode configuration, microwave losses due to impedance mismatch are most deleterious at high speeds, thereby negating the push-pull advantage. The two-electrode configuration, although not exhibiting microwave losses, has precluded push-pull operation and requires more power to effect the same modulation depth, which at high speeds, driving circuits are unable to deliver. Accordingly, prior art interferometric modulators can not be optimized for both the switching voltage and microwave coupling.